soc: ite: it8xxx2: Disable I2C0 alternate function to allow sleep entry

The chip requires a successful sleep entry to change the PLL sequence.
By default, the IT8XXX2 chip enables the I2C0 alternate function, which
can cause the clock/data lines to meet the start condition, preventing
EC from entering sleep mode.

This commit disables the I2C0 alternate function before executing the
PLL sequence change to ensure that the EC can enter sleep mode
successfully.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>